Due to their many advantages, which include high brightness and desirable spectral and angular beam characteristics, lasers are considered attractive light sources for various applications such as projection displays, microscopy, microlithography, machine vision and printing. However, one drawback to using lasers in these systems is speckle. Basically, speckle is an undesirable variation in the cross-sectional intensity of a laser beam. In laser projection systems, it usually makes images appear grainy and less sharp. Speckle is due to interference patterns that result from the high degree of temporal and spatial coherence of light emitted by most lasers. When such coherent light is reflected from a rough surface or propagates through a medium with random refractive index variations, speckle shows up as an uneven, random distribution of light intensity. This uneven brightness degrades the quality and usefulness of laser illumination systems.
The prior art describes various techniques for speckle reduction. For example, in U.S. Pat. No. 5,224,200, Rasmussen et al. propose a speckle reduction apparatus 10, as illustrated in FIG. 1. The system consists of a coherence delay line in series between a laser and a homogenizer 28. The coherence line consists of a totally reflecting mirror 24 and a partially reflecting mirror 22 separated by a distance 25 equal to an integer multiple of half the coherence length of the original laser beam. The laser beam 20 strikes the partially reflecting mirror 22 first, which transmits part of the beam and reflects the remainder toward the totally reflecting mirror 24 where it is reflected back toward the partially reflecting mirror 22. This process continues until the reflected beam bypasses the partially reflecting mirror 22. This final beam and the series of beams transmitted through the partially reflecting mirror 22 are focused by a lens 26 into a homogenizer 28. Beams entering the homogenizer 28 are offset by multiples of their coherence length, leading to a reduction in their apparent coherence length, which in turn, reduces the amount of speckle.
In U.S. Pat. No. 5,313,479 to J. M. Florence and U.S. Pat. No. 6,594,090 B2 to Kruschwitz et al., a moving diffuser is used to remove or reduce the speckle pattern.
In U.S. Pat. No. 6,897,992 B2 to H. Kikuchi, the laser beam is rotated and equally divided into its S and P polarization components. After separating the S and P polarization components, an optical path difference that is at least equal to the coherence length of the laser beam is generated between the S and P polarization components through appropriate delay means. The '992 patent also discloses an intensity separation means for dividing the laser beam into two or more parallel beamlets and delaying the beamlets relative to each other by an optical path difference that is at least equal to the coherence length of the laser.
B. Dingel et al. in “Speckle-Free Image in a Laser-Diode Microscope by Using the Optical Feedback Effect,” Optics Letters, Vol. 18, No. 7, April 1993, pp 549-551, teach a method of removing laser speckle by broadening the spectral linewidth of a laser and generating an output beam having a multimode spectrum that changes with time. This result is obtained by feeding a moderate amount of the laser light back into the cavity of the laser through the use of mirror, beam splitter and multimode fiber.
Although the above methods of speckle reduction are effective in some applications, they nevertheless suffer from one or more of the following disadvantages: moving or vibrating parts, low degree of compactness, long integration time, excessive loss of light energy (i.e., inefficiency), and/or lack of control over the spatial distribution of light in terms of angle and intensity.
Therefore, there is a need for a simple, compact, light weight, short-integration time, and efficient speckle reduction apparatus that provides control over the spatial distribution of laser light in terms of intensity and angle over a certain target area, such as the active area of a display panel.